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<title>Boost.Python - &lt;boost/python/to_python_converter.hpp&gt;</title>
<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
<td valign="top">
<h1 align="center">Boost.Python</h1>
<h2 align="center">Header &lt;boost/python/to_python_converter.hpp&gt;</h2>
</table>
<hr>
<h2>Contents</h2>
<dl class="page-index">
<dt><a href="#introduction">Introduction</a>
<dt><a href="#classes">Classes</a>
<dd>
<dl class="page-index">
<dt><a href="#to_python_converter-spec">Class Template <code>to_python_converter</code></a>
<dd>
<dl class="page-index">
<dt><a href="#to_python_converter-spec-synopsis">Class Template
<code>to_python_converter</code> synopsis</a>
<dt><a href="#to_python_converter-spec-ctors">Class Template
<code>to_python_converter</code> constructor</a>
</dl>
</dl>
<dt><a href="#examples">Example</a>
</dl>
<hr>
<h2><a name="introduction"></a>Introduction</h2>
<code>to_python_converter</code> registers a conversion from
objects of a given C++ type into a Python object.
<h2><a name="classes"></a>Classes</h2>
<h3><a name="to_python_converter-spec"></a>Class template <code>to_python_converter</code></h3>
<code>to_python_converter</code> adds a wrapper around a static
member function of its second template parameter, handling
low-level details such as insertion into the converter registry.
<table border="1" summary="to_python_converter template parameters">
<caption>
<b><code>to_python_converter</code> template parameters</b><br>
In the table below, <b><code>x</code></b> denotes an object of type <code>T</code>
</caption>
<tr>
<th>Parameter
<th>Requirements
<th>Description
<tr>
<td><code>T</code>
<td>
<td>The C++ type of the source object in the conversion
<tr>
<td><code>Conversion</code>
<td><code>PyObject*&nbsp;p&nbsp;=&nbsp;Conversion::convert(x)</code>,<br>
if <code>p&nbsp;==&nbsp;0</code>, <code><a href="http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71">PyErr_Occurred</a>()&nbsp;!=&nbsp;0</code>.
<td>A class type whose static member function
<code>convert</code> does the real work of the conversion.
<tr>
</table>
<h4><a name="to_python_converter-spec-synopsis"></a>Class template <code>to_python_converter</code> synopsis</h4>
<pre>
namespace boost { namespace python
{
template &lt;class T, class Conversion&gt;
struct to_python_converter
{
to_python_converter();
};
}}
</pre>
<h4><a name="to_python_converter-spec-ctors"></a>Class template <code>to_python_converter</code> constructor</h4>
<pre>
to_python_converter();
</pre>
<dl class="function-semantics">
<dt><b>Effects:</b> Registers a to_python converter which uses
<code>Conversion::convert()</code> to do its work.
</dl>
<h2><a name="examples"></a>Example</h2>
This example presumes that someone has implemented the standard <a
href="http://www.python.org/doc/2.2/ext/dnt-basics.html">noddy example
module</a> from the Python documentation, and placed the corresponding
declarations in <code>&quot;noddy.h&quot;</code>. Because
<code>noddy_NoddyObject</code> is the ultimate trivial extension type,
the example is a bit contrived: it wraps a function for which all
information is contained in the <i>type</i> of its return value.
<h3>C++ module definition</h3>
<pre>
#include &lt;boost/python/reference.hpp&gt;
#include &lt;boost/python/module.hpp&gt;
#include &quot;noddy.h&quot;
struct tag {};
tag make_tag() { return tag(); }
using namespace boost::python;
struct tag_to_noddy
{
static PyObject* convert(tag const&amp; x)
{
return PyObject_New(noddy_NoddyObject, &amp;noddy_NoddyType);
}
};
BOOST_PYTHON_MODULE_INIT(to_python_converter)
{
module to_python(&quot;to_python_converter&quot;)
.def(&quot;make_tag&quot;, make_tag)
;
to_python_converter&lt;tag, tag_to_noddy&gt;();
}
</pre>
<h3>Python code</h3>
<pre>
&gt;&gt;&gt; import to_python_converter
&gt;&gt;&gt; def always_none():
... return None
...
&gt;&gt;&gt; def choose_function(x):
... if (x % 2 != 0):
... return to_python_converter.make_tag
... else:
... return always_none
...
&gt;&gt;&gt; a = [ choose_function(x) for x in range(5) ]
&gt;&gt;&gt; b = [ f() for f in a ]
&gt;&gt;&gt; type(b[0])
&lt;type 'NoneType'&gt;
&gt;&gt;&gt; type(b[1])
&lt;type 'Noddy'&gt;
&gt;&gt;&gt; type(b[2])
&lt;type 'NoneType'&gt;
&gt;&gt;&gt; type(b[3])
&lt;type 'Noddy'&gt;
</pre>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
05 November, 2001
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002. All Rights Reserved.</i>